Hippocampal ß2-GABAA receptors mediate LTP suppression by etomidate and contribute to long-lasting feedback but not feedforward inhibition of pyramidal neurons.

ABSTRACT

The general anesthetic etomidate, which acts through γ-aminobutyric acid type A (GABAA) receptors, impairs the formation of new memories under anesthesia. This study addresses the molecular and cellular mechanisms by which this occurs. Here, using a new line of genetically engineered mice carrying the GABAA receptor (GABAAR) ß2-N265M mutation, we tested the roles of receptors that incorporate GABAA receptor ß2 versus ß3 subunits to suppression of long-term potentiation (LTP), a cellular model of learning and memory. We found that brain slices from ß2-N265M mice resisted etomidate suppression of LTP, indicating that the ß2-GABAARs are an essential target in this model. As these receptors are most heavily expressed by interneurons in the hippocampus, this finding supports a role for interneuron modulation in etomidate control of synaptic plasticity. Nevertheless, ß2 subunits are also expressed by pyramidal neurons, so they might also contribute. Therefore, using a previously established line of ß3-N265M mice, we also examined the contributions of ß2- versus ß3-GABAARs to GABAA,slow dendritic inhibition, because dendritic inhibition is particularly well suited to controlling synaptic plasticity. We also examined their roles in long-lasting suppression of population activity through feedforward and feedback inhibition. We found that both ß2- and ß3-GABAARs contribute to GABAA,slow inhibition and that both ß2- and ß3-GABAARs contribute to feedback inhibition, whereas only ß3-GABAARs contribute to feedforward inhibition. We conclude that modulation of ß2-GABAARs is essential to etomidate suppression of LTP. Furthermore, to the extent that this occurs through GABAARs on pyramidal neurons, it is through modulation of feedback inhibition.NEW & NOTEWORTHY Etomidate exerts its anesthetic actions through GABAA receptors. However, the mechanism remains unknown. Here, using a hippocampal brain slice model, we show that ß2-GABAARs are essential to this effect. We also show that these receptors contribute to long-lasting dendritic inhibition in feedback but not feedforward inhibition of pyramidal neurons. These findings hold implications for understanding how anesthetics block memory formation and, more generally, how inhibitory circuits control learning and memory.